The present invention relates to a method and an apparatus for cleaning flue gases with slaked lime.
In different kinds of combustion, such as combustion of fossil fuels, waste etc, flue gases are generated, which contain pollutants which are removed before the flue gases are emitted into the atmosphere. The pollutants can be solid, such as fly ash, or gaseous, such as sulphur dioxide or hydrogen chloride. The solid pollutants can be removed by means of a filter, such as an electrostatic filter or fabric filter, whereas the gaseous pollutants are removed by absorption using an absorption agent. In wet cleaning of flue gases, gaseous pollutants are removed by means of an absorbing liquid containing an absorbing agent. Gaseous pollutants, for instance acid substances as sulphur dioxide and hydrogen chloride, can also be removed by contacting the flue gases with a particulate absorbent material, which usually is slaked lime (calcium hydroxide). The contact between the flue gases and the absorbent material can suitably occur after the flue gases have left the boiler. The present invention relates to the latter so-called dry sorption technique.
In the dry sorption technique, particulate slaked lime conditioned with water is dispersed in the flow of flue gas, on the one hand the hot flue gases being cooled by the water (usually from about 150xc2x0 C. to about 70-80xc2x0C.) and, on the other hand, gaseous pollutants, such as sulphur dioxide, being absorbed by the particulate slaked lime. The particulate material is then removed from the flue gases by, for instance, filtration before the flue gases are emitted into the atmosphere. The separated particulate material, which below is referred to as powdered lime, is collected. A certain part of the collected powdered lime can be re-used in the flue gas cleaning by recirculation and addition of a small amount of unused, slaked lime. The remainder of the collected powdered lime is removed, for instance, by controlled dumping.
The slaked lime used in flue gas cleaning by the dry sorption technique is either purchased completed from a supplier of slaked lime or produced on site. Conventional production of slaked lime on site is performed in such manner that burnt lime is supplied from a storage to a dry-slaking apparatus, in which the burnt lime is slaked by adding an exactly dosed amount of water. After slaking, the slaked lime is supplied to a silo for storage. When slaked lime for flue gas cleaning is needed, it is fed from the silo through lines to a mixer, in which the slaked lime is mixed with recirculated powdered lime and conditioned with water before the mixture is contacted with the flue gases.
This conventional technique suffers from a number of drawbacks. Thus, the amount of water in the slaking of the lime must be dosed very exactly since adding more water than required for the slaking, which leaves as water vapour in the exothermal slaking reaction, causes aggregation into lumps. Such aggregation into lumps may result in blocking of the slaking apparatus and undesirable lumps in the storage silo and may interfere with or prevent feeding of slaked lime from the silo to the flue gas cleaning. A further drawback is that slaked lime has a stronger tendency to form dust and is more bulky than burnt lime. The same amount of slaked lime takes up almost twice the volume of burnt lime, which makes the slaked lime more expensive to handle and transport.
To enable handling and transporting of the slaked lime without any aggregating problems, the water content must be carefully controlled so that the slaked lime has a water content of 0-2% by weight. Such a low water content in turn causes a further problem, viz. that the slaked lime obtains a low surface area. It has in fact been established that the surface area of the slaked lime is essentially directly proportional to the water content of the slaked lime. Thus, slaked lime with a water content of 0-2% by weight has a surface area of about 15-18 m2/g, while slaked lime with a water content of 10% by weight has a surface area of about 23 m2/g and slaked lime with a water content of 15% by weight has a surface area of slightly more than 30 m2/g. Since the reactivity of the slaked lime in flue gas cleaning increases with an increasing surface area, it will be appreciated that a low water content and, resulting therefrom, a low surface area of the slaked lime result in a drawback.
One more drawback of the prior art is that the storage of the slaked lime makes it age and, inter alia, form carbonate by absorbing carbon dioxide from the atmosphere.
It will be appreciated that it would be desirable and would involve an improvement in the field if, in flue gas cleaning, it would be possible to use slaked lime with a high water content and high surface area without simultaneous aggregating problems. If also the other difficulties in storage and transport of slaked lime could be reduced or obviated, this would mean further advantages.
It has been discovered in the invention that the above difficulties in prior-art technique can be obviated by producing the dry-slaked lime in situ, i.e. in immediate connection with the flue gas cleaning, with a water content of at least 5% by weight.
According to the invention, a method for cleaning flue gases with particulate slaked lime is provided, said method being characterised in that burnt lime is first dry-slaked with water to a moisture content of at least 5% by weight, and that subsequently the slaked lime is contacted with the flue gases within 1 min of the slaking.
According to the invention, also an apparatus for cleaning of flue gases with particulate slaked lime is provided, said apparatus comprising a feeding end for feeding burnt lime and adding water, and a discharge end for discharging slaked lime, and mixing means between the feeding end and the discharge end, said apparatus being characterised in that between the feeding end and the discharge end it comprises a first section and a second section which are separated by a first overflow edge, the first section being arranged at the feeding end and comprising at least one mixing means, which is rotatable about a horizontal shaft extending in the transverse direction of the apparatus, for mixing and feeding of burnt lime and water in the longitudinal direction of the apparatus from the feeding end towards the overflow edge, and the second section being arranged at the discharge end and comprising at least one mixing means, which is rotatable about a horizontal shaft extending in the transverse direction of the apparatus, for mixing of material fed over the first overflow edge, and mixing of the material in the longitudinal direction of the apparatus towards a second overflow edge arranged at the discharge end.
Further characteristic features and advantages of the invention are evident from the following description and the appended claims.
It is preferred that the slaked lime in the invention has a water content of 5-30% by weight, more preferably 10-20% by weight. Even if water contents above 20% by weight are usable, they may cause a risk of aggregating, and water contents above 50% by weight are avoided in the invention for this reason.
The high surface area and the porosity of the slaked lime according to the invention imply that it can contain more water than conventional slaked lime and remains free flowing, i.e. the high surface area reduces the aggregating problems.
A distinguishing feature of the invention is that the slaked lime is produced in situ, i.e. in immediate connection with its use for flue gas cleaning. This condition is expressed in the invention in such manner that the slaked lime is to be contacted with the flue gas within 1 min of the slaking. Preferably, the lime is contacted with the flue gases within 30 s of the slaking. By the slaked lime being produced in immediate connection with the flue gas cleaning, the need for a storage silo and transport lines and thus also the possible problems that would be caused by aggregating in these would be eliminated. The condition that the slaked lime is used for flue gas cleaning immediately upon its production means that also problems caused by aging of the slaked line are avoided.
Even if the slaked lime according to the invention can be used as it is for flue gas cleaning, for instance when cleaning precooled flue gas from waste incineration which contains hydrogen chloride, it constitutes a particularly preferred aspect of the present invention that the slaked lime is mixed with recirculated powdered lime from the flue gas cleaning before the thus-obtained mixture is contacted with the flue gases.
In addition to the fact that such recirculation of powdered lime results in economic utilisation of the absorbent material, possible aggregating problems will be further reduced. If too much water has been added to the burnt lime in connection with the slaking, this excess of water is in fact rapidly levelled when mixing with the recirculated powdered lime. This levelling is affected to a great extent by the amount of fresh, slaked lime being very small compared with the amount of recirculated powdered lime. It is generally preferred that the proportions between slaked lime and recirulcated powdered lime are from 1:50 to 1:500, based on the weight. An addition of about 100 kg slaked lime per 10 tonnes powdered lime is more preferred.
When mixing the slaked lime with recirculated powdered lime, also a conditioning of the mixture occurs by adding water. In the cleaning of flue gases it is in fact necessary, as mentioned above, that the absorbent material dispersed in the flue gases have a certain water content, on the one hand to optionally cool the flue gases and, on the other hand, to achieve a reaction between the absorbent material and the gaseous pollutants in the flue gases. As a rule, the slaked lime should thus during mixing with recirculated powdered lime be conditioned with water so that a relative humidity (RH) of 5-60% is obtained. More specifically, in absorption of sulphur dioxide from flue gases, the absorbent material should have a water content which produces a relative humidity of about 40-60%, while the corresponding relative humidity in absorption of hydrogen chloride from flue gases is about 5-10%. These values of the relative humidity are measured in the emitted flue gases after the absorbent material has been contacted with the flue gases. The reason why the RH value is lower in absorption of hydrogen chloride is that the slaked lime forms hygroscopic calcium chloride with hydrogen chloride. To achieve the necessary RH in the cleaning of flue gases, the mixture of slaked lime and recirculated powdered lime is conditioned by adding water, which is ejected in finely divided state through nozzles over the mixture. For an optimum result in flue gas cleaning, it is important for the components to be mixed and distributed as homogeneously as possible. Different apparatus for this purpose are previously known, and an example of such an apparatus is shown and disclosed in WO 96/16 722.
As indicated above, the slaked lime should be contacted with the flue gases within 1 min, preferably within 30 s of the slaking. These times do not include the time for slaking the burnt lime which takes about 3-20 min. On the other hand, the time for optional addition of recirculated powdered lime and conditioning of the mixture with water is included. This operation should thus be carried out within 1 min at most, and is preferably carried out within a time of 10 s to 1 min, more preferred 10-30 s.
For optimal efficiency, the slaked lime in the flue gas cleaning should have a small particle size. The slaked lime preferably has a particle size of 1-10 xcexcm, more preferred 2-5 xcexcm.
As indicated above, the invention also comprises an apparatus for cleaning of flue gases with particulate slaked lime.
In contrast to prior-art dry-slaking apparatus, in which axial screw feeding or the like causes a great risk of interference and interruption in connection with aggregation, the open structure of the invention and the radial feeding in relation to the mixing means cause the material to be mixed and fed without excessive compression with the ensuing risk of clogging and interruption.
In the above-described apparatus according to the invention, it is preferred that the mixing means in the first and the second section each consist of a rotatable shaft with radial arms at the outer ends of which paddles are attached, which make an angle with the direction of rotation.
It is also preferred that the leading edge of the paddles makes an angle with the direction of rotation of 10-45xc2x0.
The cross-section of the paddles in the circumferential direction can be straight. Alternatively, the cross-section of the paddles in the circumferential direction can be curved, for instance in the form of a segment of a circle.
In the first section, it is preferred that the extent of the paddles in the circumferential direction is relatively small and corresponds to a sector with an angle of about 5-30xc2x0. On the other hand, it is preferred to have a large number of paddles in the first section, such as mixing means with a rotatable shaft having 4-12 radial arms with 2 paddles per shaft.
In the second section, it is preferred that the extent of the paddles is relatively large both in the circumferential and in the axial direction. Thus, it is preferred that the paddles have an extent in the circumferential direction which corresponds to a sector with an angle of 30-90xc2x0. The extent in the axial direction is preferably such that 2-10 pairs of paddles cover the extent of the mixing means in the axial direction.
As is evident from the description above, the inventive apparatus can be connected directly to the flue gas duct, but according to a particularly preferred aspect of the invention, the apparatus is connected to one end of a chamber with means for supplying powdered lime recirculated from the flue gas cleaning, means for adding water, and means for mixing slaked lime, recirculated powdered lime, water and discharging the mixture from the other end of the chamber into contact with the flue gases. Such a chamber can be of a construction known per se and can, for instance, consist of an apparatus according to WO 96/16 722 stated above.
With a view to further facilitating the understanding of the invention, it will, for the purpose of elucidation, be described with reference to the accompanying drawings.